(1) Field of the Invention
The present invention relates to a device for reducing the vibration generated by a rotorcraft lift rotor, and also to a rotor hub provided with such a device.
(2) Description of Related Art
The operation of a rotorcraft lift rotor, e.g. a helicopter main lift and propulsion rotor, generates parasitic forces at the head of the lift rotor. These parasitic forces then give rise to vibration that propagates to the airframe of the rotorcraft, such vibration being particularly perceptible in the cockpit of the rotorcraft.
In order to mitigate that drawback, devices are provided for attenuating the vibration that is generated, which devices are sometimes referred to as rotor head resonators.
In a first type of device, a moving mass is used together with return means suitable for repositioning the moving mass in a predetermined position.
According to document FR 2 416 838, a device is mounted on the top portion of the hub, which device comprises a moving mass that is held radially by resilient means inside a housing that is secured to the hub.
Furthermore, that mass is fastened to the top end of a rod. The rod is disposed substantially along the axis of rotation of the rotor in a recess in the rotor mast of the rotor. The bottom end of the rod is then hinged to a point that is situated on the axis of rotation of the rotor.
Thus, the resilient means tend to keep the moving mass in a rest position, the moving mass being free to move in a plane substantially parallel to the top portion of the hub of the lift rotor, i.e. in a plane that is substantially perpendicular to the vertical axis of rotation of the rotor. The moving mass then directly opposes the vibration generated by the rotor.
The forces that induce the vibration that needs to be reduced and that is generated at the rotor head can be described by using a force torsor, sometimes known to the person skilled in the art as the “rotor head torsor”. The “rotor head torsor” is then made up in particular both of three resultant forces along three mutually perpendicular axes, and of three moments about those axes, namely:
two axes referred to below and for convenience as the “first and second resultant axes” defining a plane referred to below as the “first resultant plane” that is parallel to the top portion of the rotor hub and that is thus substantially perpendicular to the vertical axis of rotation of the rotor; and
an axis that is referred to as the “third resultant axis” and that is perpendicular to said first resultant plane.
The device shown in document FR 2 416 838 is then effective in filtering the resultant forces of the “rotor head torsor” along the first and second resultant axes that are substantially parallel to the plane containing said moving mass, however it has practically no effect on the resultant force of the “rotor head torsor” along the third resultant axis that is substantially perpendicular to the axis of rotation of the lift rotor and perpendicular to the plane containing the moving mass.
Furthermore, since the moving mass used is constant, the first device is particularly effective when the excitation frequency of the vibration that is to be reduced is close to the resonant frequency of said moving mass, which resonant frequency is constant. The first device with its concentrated mass is then not very well adapted to excitation frequencies that vary.
Document FR 2 749 901 describes a device having a main moving mass that is held radially inside a housing. It is also provided with an adjustment moving mass suitable for sliding on the rod that is fastened to the main moving mass and that is hinged to a point situated on the axis of rotation of the rotor.
By moving the adjustment moving mass, it then becomes possible to adapt the device so as to enable it to reduce vibration at frequencies that vary. Nevertheless, the absence of filtering of the resultant forces of the “rotor head torsor” along all three mutually perpendicular axes remains, and in particular the absence of filtering along the third resultant axis that is substantially parallel to the axis of rotation of the lift and possibly also propulsion rotor.
Document FR 2 575 800 describes a vibration damper for a helicopter rotor hub, which damper is provided with a central moving mass suspended inside an outer casing along three directions by mechanical and pneumatic springs.
Thus, the same mass is used to combat all three resultant forces of the “rotor head torsor”.
Although effective, it is found that the stresses along the third resultant axis differ from the stresses along the first and second resultant axes. Developing such a device can therefore be difficult. Furthermore, reducing vibration at varying frequencies can be complex.
In the same manner, document EP 0 790 180 provides for suspending a central moving mass from a plurality of springs, the central moving mass also being arranged on a movement limiter that extends along a vertical axis.
Finally, document U.S. Pat. No. 6,443,273 discloses the possibility of suspending a central mass in a casing by using springs.
It should also be observed that document EP 1 007 406 describes a spring-mass type device.
Devices of the first type appear to be difficult to adapt to counter forces along all three resultants of the “rotor head torsor” when generated by vibration at varying frequencies.
In a second type of device, weight elements are distributed around the axis of rotation of the rotor.
Document WO 2005/079200 describes a first variant of the second type that is provided with two coaxial weight elements together with control means, the control means being suitable for controlling the speed of rotation of said weight elements and their relative angular position.
In a second variant of this second solution, pendulums are used that oscillate under the effect of centrifugal force. Each pendulum has a weight element connected to a hinge of a support, the support performing rotary motion about the axis of rotation of the main rotor. For example, the support is provided with a plurality of radial arms forming a star, each radial arm having a hinge connected to a weight element.
Under such circumstances, and unlike devices of the first type, there is no need to implement dedicated return means, centrifugal force providing the required return force.
That feature gives pendulum devices a capacity to adapt automatically to variations in the frequency of the vibration for attenuating.
A change in the speed of rotation of the rotor gives rise not only to a change in the frequency of the vibration for attenuating, but also gives rise to a change in the centrifugal force exerted on the pendulum. Thus, those devices of the second type are said to be “self-adaptable” or “self-adjustable”.
The pendulums can oscillate in a first plane perpendicular to the axis of rotation of the main rotor, or in a second plane having said axis of rotation of the main rotor lying therein, as a function of the nature of the vibration to be attenuated.
In a first embodiment of a pendulum device, referred to as a “simple pendulum resonator” for convenience, the weight element performs circular motion about a single axis of rotation.
Documents FR 2 733 483 and FR 2 435 391 describe such simple pendulum resonators.
In a second embodiment referred to as a “bifilar pendulum resonator” by the person skilled in the art, each weight element moves in circular translation. The term “circular translation” is used of a body that is moving in a plane, with two distinct points of said body describing two circular paths having the same radius but different centers.
Compared with a simple pendulum resonator, the bifilar pendulum resonator is capable of filtering higher frequencies.
Document FR 2 018 491 describes such a second embodiment.
Each bifilar weight element comprises a U-shaped member having a branch of a star-shaped support engaged therein. The U-shaped member is provided with two first openings of circular section that co-operate with two second openings of circular section in a support via two rollers.
Document FR 2 768 995 describes a third embodiment of a device of the second type referred to as an “accelerated pendulum resonator”, for convenience. According to that document, the weight element is a counterweight connected by a connection arm to a support branch, the support not being secured to the hub of the rotor, but rather to a drive member that rotates at a speed of rotation that is faster than the speed of rotation of the rotor.
Independently of the embodiment, devices that implement pendulums do not appear to be sufficient to counter all of the forces that result from the “rotor head torsor” along three mutually perpendicular axes, and more particularly the resultant force along the third resultant axis that is substantially parallel to the axis of rotation of the lift rotor.